BoilerBuddy · guided walkthrough

One radiator cold at the bottom — bleed or sludge?

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A guided self-check for the classic “one radiator part-cold while the others heat fine” symptom. Feel both ends, find the bleed valve, attempt a bleed. If it works, re-pressurise and you are done. If the bleed signature points to system sludge, hand off to a Gas Safe engineer with the symptom already written up.

4 steps · one tap each
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Before you start

What you need on the radiator.

You will be at the radiator the whole time. The walkthrough tells you exactly where to look and what to do at each step.

  • The cold radiator

    Stay with the radiator for the whole walkthrough. You will feel the top and the bottom, look at the bleed end, and run the bleed a quarter turn at a time.

  • A radiator key

    A small square or slotted metal key that fits the bleed nipple. Most bleed valves take the standard radiator key; some take a flat-blade screwdriver on the square drive.

  • A cloth

    A dry cloth to catch the drips. Have it ready before you crack the bleed valve — water will appear within a few seconds and you want to close the valve the moment it does.

Walkthrough

Tap the option that matches what you see.

Each step has a clear “yes” and a clear “no / stop”. If anything looks wrong, tap the option that says so — the walkthrough will route you to a Gas Safe engineer route with the symptom already written up.

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Fetching the first step.

Where the DIY line stops

Stop and call a Gas Safe engineer if any of these apply.

  • The bleed valve will not turn or feels seized — do not force it, the casting can crack.
  • The bleed released water with no air, only air, or nothing came out at all on a top-hot/bottom-cold radiator — that is the system-sludge signature.
  • Both ends of the radiator are uniformly cold — no flow is reaching this radiator, a bleed cannot fix that.
  • The cold-at-the-bottom patch returned within a few days of a successful bleed — that is the system-contamination signature, not a fixable-by-bleed issue.
  • The boiler is not firing or has no hot water anywhere — this guide is for a single radiator, not whole-system faults.
  • You smell gas anywhere in the property — STOP, leave the property, and call the Gas Emergency line.
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